Capsaicin, N-vanillyl-trans-8-methyl-6-nonenamide, is a hot substance found in the fruit juice of a plant belonging to the genus Capsicum. U.S. Pat. No. 4,313,958 issued to LaHann, and European Patent No. 0 282 127 granted to Garnder, disclose as having analgesic activities such natural capsaicin and synthetic capsaicin derivatives, e.g., N-{4-(2-aminoethoxy)- 3-methoxybenzyl}amide derivatives.
Also reported in LaHann et al., U.S. Pat. No. 4,424,205; Gardner et al., U.S. Pat. No. 5,045,565; and Park et al., U.S. Pat. No. 5,242,944 are various homovanillic amides, i.e., phenylacetamide derivatives, as having pharmacologically and physiologically activities superior to the conventional capsaicin derivatives. However, each of thoese disclosed in the prior art still exhibits some of the skin irritability and toxicity unique to a capsaicin.